gearbox/speedometer trouble

[59gmc]

I am in need of help as my speedometer does not work in my 59 GMC stepside3 speed manual.
I have traced the problem to the gearbox .
The speedo cable and the drive that goes into the gearbox are in good working order.Both when manually rotated do make the speedometer work.
I removed the speedo drive from the gearbox and on feeling inside was unable to find the gear or cog that would rotate the speedo drive unit.
The shaft that would normally hold that gear or cog does turn when the prop shaft turns.
Has anyone got a diagram of the internal workings of the rear section of the gearbox that holds the speedometer cable and drive unit?
This would greatly assist me in removing that section from the vehicle.
Many thanks for reading